Thermal imaging, where localised heating is used to image a heat-sensitive material, has found widespread acceptance in the imaging industry. Most heat-sensitive materials generally fall into two main classes:
(1) Direct imaging materials where imagewise heating of the material modifies a heat-sensitive component of the material in the heated areas, e.g., to produce a change in colour, typically from colourless to black. Because of their ability to form coloured images by the simple application of heat, such heat-sensitive materials are widely used, not only for copying books and documents, but also for recording output information from computers, facsimile apparatus, telex and other information transmission apparatus. Direct imaging materials usually consist of a single sheet. PA1 (2) Thermal transfer materials where the desired image is formed on a receptor by the transfer of a heat-activated colourant from a donor sheet. Thermal transfer materials include both mass-transfer and diffusion-transfer materials, the latter sometimes referred to as sublimation-transfer or dye transfer materials. In each case, the donor sheet typically comprises a paper or polymeric film carrier sheet having a heat-activated, image-forming layer (referred to as the "donor layer") containing the colourant on its front or top surface. The term "colourant" is used in its broadest sense as covering any material capable of modifying the surface of the receptor, regardless of whether the modification is visible to the naked eye and would include dyes, pigments, waxes, resins etc. The colourant usually comprises one or more dyes or pigments with or without an additional binder. PA1 (a) passing the magnetisable recording member and a flexible prerecorded magnetisable medium through a nip of two rolls, one of said rolls being transparent to the beam of energy and facing the recording member and at least one of said rolls being compressible, the chromium dioxide particles facing said prerecorded magnetisable medium, PA1 (b) passing a continuous beam of energy through the transparent roll and the flexible support of the magnetisable recording member to simultaneously heat chromium dioxide particles across a width of the recording member on which a magnetic particle (sic-pattern?) is to be replicated for a time period in the range 0.01 to 5 milliseconds while in said nip and in contact with the prerecorded magnetisable medium, said chromium dioxide particles being heated to above their Curie temperature, PA1 (c) cooling the chromium dioxide particles to below their Curie temperature while in intimate contact with the prerecorded medium in said nip for a time period in the range from 0.1 to 100 milliseconds.
Diffusion-transfer materials depend on the colourant migrating under the action of heat across the space between the donor sheet and receptor and condensing on the latter. In use, the donor sheet and receptor are assembled (but not permanently bonded) in intimate, face-to-face contact and held under pressure so that the receptor is close enough to receive the transferred colourant. The applied pressure ensures that the space separating the donor sheet and receptor remains constant, thereby ensuring that the transferred spots are of equal size. The amount of colourant transferred is proportional to the intensity of the energy absorbed, thereby giving a continuous tone image.
Mass-transfer materials depend on either the colourant layer on the donor sheet, or a suitable receptor layer on the receptor sheet, being momentarily softened or melted by the application of heat such that, on solidification, the colourant layer adheres to the receptor and remains attached thereto on separation of the donor and receptor. Unlike diffusion-transfer materials, either 0 (zero) or 100% transfer of colourant takes place depending on whether the absorbed energy exceeds a threshold value. In order to achieve high sensitivity, it is essential that the donor sheet and receptor are pressed tightly together to ensure efficient use of the heat applied.
In both direct and thermal transfer imaging systems, a thermal printhead comprising an array of miniature electrically-heated elements, each of which is capable of being activated in a timed sequence to provide the desired imagewise pattern of heating, is contacted with the heat-sensitive material. Pressure is applied to the heat-sensitive material, e.g., by a roller, to pinch the material against the heating surface of the printhead, and the printhead selectively activated to heat the material in an imagewise fashion.
In thermal transfer imaging, the image-forming layer of the donor sheet is placed in intimate contact with the surface of the receptor and the back or opposite side of the donor sheet contacted with the thermal printhead. Pressures of greater than 15 g/mm.sup.2 are typically applied to the assembled donor sheet and receptor in order to pinch the sheets against the heating surface of the printhead. With some systems, pressures of up to 50 g/mm.sup.2 or higher may be required for efficient colourant transfer. During imaging, the donor sheet may be exposed to temperatures of up to 300.degree. C. or higher for short periods of time in order to effect colourant transfer.
To provide a reasonable image quality, the printheads require a high density of electrically-heated elements, which must be both accurately sized and of uniform resistance. A number of highly accurate microlithographic fabrication stages are required to achieve this. The requirement that all the elements must be functional and of uniform resistance at this level of fabrication complexity leads to a low yield. The cost of the thermal printhead is also high.
The use of thermal printheads to image thermal transfer materials is also found to provide rather poor resolution and increasing interest is being shown in the use of radiant or projected energy, especially infrared radiation, to supply the heat, thereby taking advantage of the greater commercial availability of lasers emitting in the near-infrared region of the spectrum. This is achieved by incorporating a radiation-absorber in one of the donor sheet and receptor and subjecting the assembly of donor sheet and receptor to an imagewise pattern of radiation. When the donor-receptor assembly is irradiated by radiation of an appropriate wavelength, the radiation-absorber converts the incident energy to thermal energy and transfers the heat to colourant in its immediate vicinity, causing imagewise transfer of the colourant to the receptor.
Another type of thermal imaging is the so-called Thermal Magnetic Duplication (TMD), whereby information stored on a magnetic recording member (e.g. a video type) is duplicated on a second magnetic recording member by maintaining the recording media of the respective members in face-to-face contact while the second magnetic recording medium is heated above its Curie temperature. This technology is described, for example, in U.S. Pat. Nos. 4,907,103, 4,698,701 and 4,631,602. A laser or other radiant IR source may be used to effect the heating, and by focussing it on the second magnetic recording medium, selective heating of the latter may be achieved by absorption of the radiation.
When using radiant energy to image heat sensitive materials, e.g. by means of a scanning laser, the problem arises of simultaneously applying sufficient pressure and maintaining accurate focus of the laser whilst scanning the media. Various designs of laser scanner are available, including flat-bed, internal drum and external drum scanners. Typically, these require expensive optics to keep the laser in focus, and employ vacuum hold-down to secure the media (see, for example, U.S. Pat. Nos. 5,053,791 and 5,017,547). The maximum pressure theoretically available by this means is 14.7 psi (10.3 gmm.sup.-2), but considerably less is achieved in practice, and this is found to be insufficient for many types of thermal transfer media.
U.S. Pat. No. 4,470,055 discloses an inking drum comprising a transparent hollow roller having coated on the external surface thereof a transparent electroconductive layer and a photoconductive layer. A semiconductive ink which is solid at room temperature is coated onto the drum and a receptor sheet brought into contact with the ink layer. With a voltage applied between the ink layer and the transparent electrode, illumination from within the drum causes the photoconductive layer to switch to a low resistance state in the exposed regions generating heat which causes fusion and transferral of ink to the receptor sheet.
Our copending European Patent Application No. 90308954.8, filed 15th Aug. 1990 discloses a thermal imaging assembly comprising a transparent support shaped as a hollow drum, having coated on the external surface thereof (in order of coating) a transparent electrically conductive layer, a photoconductive layer and a second electrically conductive layer which may or may not be transparent. An inking station coats the drum with a layer of colourant containing medium, e.g., a paste or jelly impregnated with ink. In use, a receptor sheet is brought into contact with the inked drum and a voltage applied across the electroconductive layers. When exposed to light source mounted internal to the drum, the photoconductive layer switches to a low resistance state in the exposed regions, generating heat which causes transferral of colourant to the receptor sheet.
U.S. Pat. No. 4,959,663 discloses a thermal printhead comprising a transparent support bearing a photoconductive layer and electrodes for applying a voltage across the photoconductive layer. The photoconductive layer is exposed through the support using a laser intensity modulated according to image information. The change in resistance of the photoconductive layer in the exposed regions generates heat which is used to image a heat-sensitive material held against the printhead. In one embodiment, the support is formed as a cylindrical rod which focuses the laser beam onto the photoconductive layer.
In each of the aforesaid devices the exposure source is focused onto a photoconductive layer, with heat generated as a result of the current flow caused by the switch to a low resistance state in the illuminated regions.
U.S. Pat. No. 4,631,602 describes a process for replicating a magnetic pattern on to a moving flexible magnetisable recording member containng a particulate layer with chromium dioxide particles on a flexible support transparent to a beam of energy which comprises
In the only embodiment described, the transparent roll takes the form of a hollow drum, the beam of energy being directed from within the drum, in which the drum exerts no focussing action on the energy beam.
This process is broadly similar to the mode of action of overhead transparency makers, in which an original bearing an IR-absorbing image and a transparent, heat-sensitive copying sheet are assembled in intimate contact and fed between two rollers, one of which is a transparent drum containing an internal radiation source. The source is typically an incandescent source rather than a laser, but again no focussing action is exerted by the drum.
British Patent No. 1184820 discloses apparatus for reading information on punched cards, sheets, tapes and the like, in which the card or other record medium is passed between two rollers, each roller comprising a cylindrical "core" extending between two end mounted discs. The end discs have a larger diameter than the core portions such that there is no contact between the core portions of the two rollers. The core portion of at least one roller is formed of a transparent material.
In use, the leading edge of the card or other record medium to be read is driven by the cooperating end discs between the space formed between the core portions of the rollers and immediately above a bank of photocells. Light from an exposure source mounted above the transparent core is focused or otherwise transmitted by the core onto the card. The pattern of light passing through the holes in the card is sensed by the photocells which produce a corresponding electrical signal.
Su-1352439 discloses a microfilm copying apparatus comprising a cylindrical lens which is biased against a drive roller by the action of two equi-sized clamping rollers. The microfilm and film are passed between the pinch formed by the lens and drive roller. Light from an exposure source mounted above the clamping rollers is focussed by the lens onto the contacted film and microfilm sheets to expose the former.
The use of cylindrical lenses as part of the collimation system for laser scanners is also known. For example, the output radiation from laser diodes is collimated with conventional lenses to produce a beam with an elliptical cross-section. A cylindrical lens is sometimes used to selectively refract one axis of the beam so that it can be focused to a circular spot which is the preferred shape for use in imaging systems.
The present invention seeks to provide an alternative method of imaging heat-sensitive materials by exposure to a source of radiant or projected energy.